Construction of Battery Charge Control for Photovoltaic Systems
1.0 Background of the study
A charge controller is an essential part of any alternative energy system. In its simplest form, a charge controller’s job is to make sure the power (such as a solar panel) ‘plays nice’ with the load (such as a battery). The simplest implementation of this is a single diode placed in between a solar panel and battery. This ensures that the battery does not discharge into solar panel at night. A more sophisticated implementation will be adding the ability for the charge controller to disconnect the solar panel when the batteries are fully charged in order to prevent over-charging damage to the batteries (James and Dunlop, 1991).
The current version of the open source free charge controller is a converter for charging batteries, A bulk converter steps down voltage from a higher voltage level to a lower voltage level. In this case, it would step voltage down from the 18volts of a solar panel to the 12volts of a battery (Harrington and Dunlop, 1992). Since the converter is software controlled, it can be programmed to charge any battery chemistry, change it drive frequency to achieve maximum conversion efficiency as well as implement MPPT to allow a solar panel to deliver maximum power, all without any changes to hardware (Robert and Isaac, 2007). A charge controller also called charge regulator or battery regulator limits the rate at which electric current is added to or drawn from electric batteries. It prevents over charging and may prevent over voltage, which can reduce battery performance or life span, or may pose a safety risk. It may also prevent completely draining (deep discharging) a battery, or perform controlled discharges, depending on the battery technology, to protect battery life.
The term charge controller or charge regulator may refer to either a stand-alone device, or control circuitry integrated within a battery pack, battery-power device, or recharger (Dunlop, 1991; Harrington and Dunlop, 1992) Basically, there are four types of charge controllers. These are namely:-
I. Series charge controller or series regulator
II. Shunt charge controller or shunt regulator
III. Pulse width modulated charge controller (MPPT)
IV. Maximum power point tracker (MPPT)
A series charger controller disables further current flow into batteries when they are full. A shunt charge controller diverts excess electricity to an auxiliary or shunt load as electric heater, when batteries are full (Harrington and Dunlop, 1992). Pulse width modulated (PWM) and maximum power point tracker technologies adjust charging rate depending on the battery voltage level to allow charging closer to its maximum capacity. Charge controller may also monitor battery temperature to prevent over-heating. Some charge controller systems also display and transmit data to remote displays and data logging to track electric flow over time (Sanjit, 1980; James and Dunlop, 1991; Robert and Isaac, 2007).
The primary function of a charge controller in a stand-alone PV system is to maintain the battery at highest possible state of charge while protecting it from over charge by the array of solar panels and from over discharge by the loads, Although some PV system can be effectively designed without the use of a charge control, any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost) typically requires a battery charge controller (James and Dunlop, 1991). The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and most importantly the ability of the system to meet the load demands. Additional features such as temperature compensation, alarms, meters, remote voltage sense leads and special algorithm law enhance the ability of a charge controller to maintain the health and extend the lifespan of battery, as well as providing an indication of operational status to the system caretaker (James and Dunlop, 1991; Harrington and Dunlop, 1992).
1.1 AIM AND OBJECTIVES
The project is aimed at the following:-
I. Preventing battery overheating to limit the energy supplied to the battery by the PV array when the battery becomes fully charged.
II. Preventing battery undercharge to disconnect the battery from electrical loads when the battery reaches a low state of charge.
III. Providing load control functions to automatically connect and disconnect an electrical load at specified time, for example operating a lighting load from sunset to sunrise.
IV. Designing a control algorithm for charge controller to determine which particular algorithm will be suitable and efficient for charge regulation (Stevens, 1999, Robert and Isaac 2007)
V. Knowing how to design, select and match guidelines for battery application and charge control requirements in PV systems.
1.2 JUSTIFICATION OF THE STUDY
Photovoltaic systems remain the best alternative to the power supply problem in Nigeria today. And the important of a charge controller in a stand-alone photovoltaic system cannot be over emphasized as mentioned earlier. But the efficiency of a charge controller clearly depends of the regulation technique that is used. Modern charge controllers employ the dynamic potentials of pulse width modulation in tracking the maximum power of the battery bank, through the voltage regulation set points. This method provides for a range of voltages through which charge disconnection and reconnection occurs. However, the type of charge controller described above makes use of several integrated circuits (ICs) to generate the pulses. This is usually complex and expensive to realize. In this project work, a single chip of IC 555 timer is used to generate the pulses, bearing in mind its basic function as a multivibrator, its availability and low cost.
The project begins with the sourcing of materials and textbooks in the library and through the internet, to understand what a charge controller is and how it works including it’s basic functions. The research continued with an inquiry into the various types of charge controllers with their respective charge regulation techniques and designs which includes series design, shunt design, pulse width modulation and maximum power point tracking. Thereafter, the design of the project was done to determine the appropriate components to be used. Thus, linear circuit theorems were applied for the mathematical analysis of various voltages, currents and resistances at set points. For a 24v, 0.96Kw charge controller, it will handle a maximum current, I as follows:
From P = V2/R,
where P represents total power,
V represents voltage and
R represents circuit resistance,
If P = 0.96kw and V = 24v,
then 0.96 = (24)2/R or R = 24×24/960 = 0.6 ohms
By ohm’s law, V = IR,
where I represents the maximum current, it implies that
I = V/R = 24/0.6 = 40A
Therefore, the maximum short circuit current that can be managed by the charge controller is 40 amperes. Other theorems such as The venin’s and Norton’s theorems are also applied in addition to the modeled equations to calculate and determine the values of components used. After the designing, the components are sourced according to the design specifications made by calculation. Thereafter, the components are mounted by carefully following the schematic diagram for the project. The project work is tested to examine how it functions, and a charge controller algorithm is designed to operate the charge controller according to the voltage regulation set points. The last stage of the project work deals with conclusion, where emphasis is laid on problems encountered during the process of carrying out the project work and suggested solutions as well as recommendations on the need to encourage charge control in solar energy systems.
1.4 EXPECTED FINDINGS
At the end of the project it is expected that:-
I. In a PV system, the battery system should be electrically designed for optimal performance and safety
II. Adequate knowledge of different types and classification of battery charge controller will be acquired.
III. Different design and hardware assembly for the various types of charge controller can be achieved.
IV. There is new technique in getting the maximum power by using the pulse width modulation technique; the PIC micro controller performs this function primarily. But it is believed that 555timer IC can also perform this function due to its characteristics (Sanjit, 1980; Lam, 1995)
1.5 EXPECTED CONTRIBUTION TO KNOWLEDGE
I. The project work will give an insight of the common terminologies associated with battery charge controller in PV systems.
II. Detailed understanding of the actual modes operations of different charge controllers in PV systems will be enhanced.
III. The effect of charge rate, charge regulation algorithm and set points on battery performance and life in PV systems will be clearly understood (Robert and Isaac, 2007).
The importance of equalization for batteries in PV systems will be known in addition to knowing the types of equalization and conditions necessary for adoption of battery equalization (Lam, 1995)
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